1. Field of the Invention
The present invention relates generally to an electroacoustic transducer and more particularly to an electroacoustic actuator and till more particularly to a receiver or loudspeaker for use in hearing aid applications.
2. Description of the Prior Art
An electroacoustic transducer for use in a hearing aid is known in the prior art for instance from GB-A-2.018.089. The transducer disclosed therein makes use of a moving tongue which is moved by an electrical current flowing through a coil of wire and generating a magnetic field. The tongue is placed in a static magnetic field and the electrically generated magnetic field causes movement of the tongue. The tongue is used to displace a mechanical piston or diaphragm thus creating a sound pressure. The conversion of input electrical energy into audible sound pressure energy is thus via magnetic field and mechanical force. The transducer shown in GBA-2.018.089 comprises an acoustic space and two opposite transducer surfaces or membranes moving in mutually opposite directions in use. A problem inherent in this electromagnetic type of prior art receiver resides in the fact that it is relatively complex to produce since it is made up of a considerable number of components. It is not sufficiently sturdy and is often damaged if the hearing aid, in which the transducer is incorporated, is dropped at the floor. Furthermore, it constitutes a major limitation for a good acoustic operation of a hearing aid.
As an alternative to the driving means in the form of a movable tongue the transducer surfaces in the above mentioned GB-A-2.018.089 may be plates of a condenser or ceramic or electret or piezoelectric polymer transducer. The arrangement of a transducer of this type has not been disclosed in the patent.
DE-A1-3.309.851 discloses a transducer membrane for an electroacoustic transducer in which piezo ceramic layers provided with electrodes on both sides are placed close to both sides of the membrane and electrically parallel connected with a view to compensate for temperature dependency, in particular when used in telephones. This prior piezo structure is designed for use in an ordinary telephone and has not been optimized for hearing aid applications.
EP-B1-0 146 933 discloses an alarm sound generator or buzzer for automobiles which comprises a housing with an internal air chamber between two membranes carrying respective piezoelectric elements. Such piezoelectric devices are limited to generation of narrow bands of frequencies as they rely on operating at one or a few resonances.